highPass: SPM high pass filter
In FIACH: Retrospective Noise Control for fMRI

Description Usage Arguments Value Author(s) Examples

Removes low frequency drifts from data.

1	highPass(x, freq, tr)

`x`	Numeric matrix or vector. If a vector is supplied it will be coerced to a matrix.
`freq`	This is the cutoff frequency. It is specified in seconds for comparability with SPM.
`tr`	This is the time between samples(or the TR for fMRI data.)

A high pass filtered matrix will always be returned.

Tim Tierney

t<-seq(1,300)
y<-cos(.01*pi*t)+.1*cos(.2*pi*t)   ## time series with low frequency drift
plot(t,y,type="l",col="red",lwd=5) ## plot it
hp.y<-highPass(y,128,2.16)        ## filter it at 128s (like in SPM) with a tr of 2.16
lines(t,hp.y,col="blue",lwd=5)     ## result has no drift
legend(x="topright",               ## Create the Legend
legend=c("Raw Signal",
         "Highpass Filtered"),
col=c("red","blue"),lwd=2)